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  In comparison to the use of the captive bubble method in the measurement of contact angles in other cases, the contact angle in the study of lung surfactant monolayer is kept at a constant 180 degrees, due to the property of a hydrated agar gel of the ceiling of the bubble. The system applied in the study of lung surfactant is designed to be a leak-proof system, ensuring the independence of the surface film of bubbles from other materials and substances like plastic walls, barriers, and outlets. Instead of adding extra tubing or piercing the bubble air-water interface with needles, this closed system is created by adjusting the pressure within the closed sample chamber by adding or removing aqueous media to regulate the bubble size and surface tension of insoluble films at the bubble surface.
  Since the bubble volumes are controlled by modifying the pressure in the sample chamber, the surface area and the surface tension of the surfactant film at the bubble surface are reduced as the volume of the bubble decreases.
  The bubble shape, in this case, can vary from spherical to oval shape depending on the surface tension, which can be calculated through the measurement of height and diameter of bubbles. In addition to measuring the surface tension, bubble formation can also be utilized in the measurement of the adsorption of lung surfactant, which defines how quickly substances build up on the air-liquid interface of pulmonary surfactants to form a film.
  There are two methods to measure adsorption with captive bubbles:
  # One method of forming bubbles to measure adsorption is to begin with a small bubble of a diameter of 2–3 mm in a chamber with a diameter of 10 mm then expand or compress it later. The bubble is first introduced into the chamber with a small plastic tubing attached to a 50 µl micro-syringe. It is then expanded through a sudden decrease in pressure inside the captive bubble or an increase in chamber volume by moving the piston on the end of the glass cylinder. To calculate the exact adsorption rate, the initial amount of surfactant on the bubble surface before volume modification has to be taken into consideration.
  # Another method of measuring adsorption is to start a bubble with a fixed volume instead of a given size or diameter by utilizing a needle on the bottom inlet of the bubble chamber. The fixed volume to start with is usually 200 ml, which is around 7 mm in diameter. Just as the condition for the first method, the accumulation of material on the bubble surface during bubble formation has to be calculated in order to evaluate the exact rate of adsorption.